how to alter a subpartition

how to add extents for subpartitions. Like this case:

OWNER SEGMENT_TYPE

------------------------------

SEGMENT_NAME

----------------------------------------

XXX TABLE SUBPARTITION

XX_TEST_EXTENT_LOG_PART

+++++++++++++++++++++++++++++++++++++

Query below does not help in adding extents to subpartitions

SQL> alter table test_extent allocate extent(size 5m); (adds 5 extents to the table)

Thank you, Ramesh